Hi sashi bhusan,
thank you for reaching out support , I will help you on cache refresh issue, So over the sourceBackend we given the information of servers whose information need to cache and update locally.
There is also button know as test LDAP connection , please make sure your connection to ldap server works.
Also please let me know which document you are following and also share the screenshot of all the tabs of cache refresh configuration.
Thanks and Regards